It is unlikely that many would be leaking given that wells are only abandoned once reservoir pressure becomes insufficient to produce a well. And even after this, more oil is usually extracted using artificial drivers like gas re-injection, or gas lift. So at the point of abandonment it would take a pretty big change in reservoir conditions to cause a leak. In theory though I guess it is possible that some sort of geological change could cause this, and perhaps the seals could also lose integrity as they age, so it would probably be a good idea to have some sort of maintenance / checking program where abandoned wells are randomly inspected for leaks.

Close, but not always the case. Operators abandon wells when they are no longer profitable. So, when a formation stops producing valuable oil or gas, the operator has the well plugged and leaves. Unfortunately, the field may still have plenty of pressure and plenty of non-valuable fluids remaining and thus a potential for leakage.

The anonymous poster above is also partially correct. An ‘abandoned’ well is supposed to be properly plugged by filling it with concrete (its more complicated than that). However, these things are tough to regulate when pluggers are trying to eek out every penny when the inspection team isn’t looking.

This statistic is meaningless! An abandoned well is one that has been permanently plugged with cement and fully sealed. Abandoned wells are not wells that the owner has “walked away from”. In some states, those are called orphan wells. On the Outer Continental Shelf, where the Minerals Management Service regulates, there is strict supervision of the plugging process. In shallower water, the states regulate the condition of wells. Again, the states of Texas, Louisiana, Mississippi, and Alabama have very strict supervision of the abandonment process.

In no way are those wells in danger of leaking. Pennsylvania is full of plugged and abandoned wells dating from the 1880s and there isn’t a huge outcry over environmental problems from old abandoned wells. They simply aren’t a problem.